LOUISVILLE, Ky. – Ken Ham, the creationist founder of the Ark Encounter in Northern Kentucky, is sparring with a national group over whether public schools are legally allowed to visit his religious attractions.

Earlier this month the Freedom From Religion Foundation, which promotes the separation of church and state, sent letters to more than 1,000 public school districts in Kentucky and four other states saying that field trips to Ham's Ark Encounter and Creation Museum are unconstitutional.

The letters, sent Jan. 8, were prompted by Ham encouraging public schools to visit his theme park, which features a 510-foot-long model of Noah's Ark.

"It is unacceptable to expose a captive audience of impressionable students to the overtly religious atmosphere of Ham’s Christian theme parks," wrote Annie Laurie Gaylor and Dan Barker, co-presidents of the Wisconsin-based foundation.

Ham is the founder of Answers in Genesis, a Christian creationist ministry that runs the Ark Park in Williamstown and the Creation Museum in Petersburg. Creationists reject the teaching of evolution and believe the Earth was created in a few days about 6,000 years ago, based on the Bible's teachings in Genesis.

The Creation Museum and the Ark Encounter show dinosaurs and humans living alongside each other. According to the U.S. Geological Survey, dinosaurs went extinct nearly 65 million years before people appeared on Earth.

The same day the letters were sent, Ham fired back by offering free admission to any public schools that take students on official field trips to the Ark Encounter and Creation Museum.

"And if the FFRF dares threaten or bully a public school, we have access to expert constitutional law attorneys who will provide their services to the school, pro bono, even if that means going all the way to the US Supreme Court," Ham wrote in a blog post.

A phone call to the Ark Encounter seeking comment was not immediately returned Friday night.

This isn't the first time Ham and the Freedom From Religion Foundation have gone head-to-head.

The foundation was one of several groups outraged when the theme park won more than $18 million in state tax incentives when it was being built in 2014. State officials tried to take the tax break away after learning the park would only hire Christians, but park officials sued in federal court and won.

After its first year, the Kentucky government cut the Ark Encounter a rebate check for $1.8 million.

The foundation's letter – sent to 1,273 public school districts in Kentucky, Indiana, Ohio, West Virginia and Illinois – said Ham "has been clear about the proselytizing nature" of the attractions.

In a June 2018 blog post, Ham wrote "the whole purpose of building these attractions was evangelistic. ... We make no apology for the fact that AiG is an evangelistic, biblical-authority ministry."

Andrew Seidel, the foundation's director of strategic response, told the Courier Journal the group wants public schools "to understand that taking other people's children to an amusement park that is dedicated to proselytizing for Christianity" violates the First Amendment.

"I don't want to drag a public school district into court over this issue," Seidel said. "We're just trying to make sure they understand."

Several legal experts told the Courier Journal that courts, citing the Constitution's Establishment Clause in the First Amendment, have made clear that public schools cannot force religion on students.

However, the purpose of field trips to religious-themed places like the Ark Encounter is also key, experts said.

University of Louisville law professor Russell L. Weaver said if a field trip to the Ark Encounter is taken as part of a class on religion, and no attempts are made to convert students, then it may pass legally.

"Context, purpose and motive are all essential in evaluating whether something like this is an inappropriate endorsement of religion or not," added professor Steve Sanders, who teaches constitutional law at Indiana University.

Corey Shapiro, legal director of the American Civil Liberties Union of Kentucky, said courts have also made clear that "public schools may not teach creationism or intelligent design – theories which the Ark Encounter espouses."

"A public school field trip to this establishment is an unconstitutional government promotion of religious beliefs, which have no place in our public schools, and improperly proselytizes to students," Shapiro said in a statement.

Seidel, with the Freedom From Religion Foundation, said it does not matter if such field trips are deemed "voluntary" or allow students to opt out, as courts have routinely rejected such arguments.

Seidel said the nonprofit has received complaints from parents and community members in the past about trips to the Ark Park and has stopped several local parks and recreation departments from taking publicly funded trips to the theme park.

At least one superintendent in Kentucky isn't fazed by either argument.

Rick Ross, superintendent of the Mason County School District, said while there have been no plans to visit the theme park, he's not intimidated by the foundation. Ross said he took the foundation's letter and put it in the trash.

We received the letter. It went straight to the garbage. I’d listen to a teacher planning a parent approved trip to either the Ark or creation museum.

"The idea of an agency outside of the state threatening us more or less, it just doesn't sit well with me," Ross said.

Ross said feedback from parents and the school district's attorney "would be vital" in deciding on any field trip to the Ark Encounter or Creation Museum.

"The letter itself would not be enough to deter us, but we are a long way from approving any trip at all," Ross said. "We would have to make sure that the purpose of the trip is to educate, not indoctrinate."